The injury sustained by Manchester United defender Luke Shaw was caused by too many training sessions from new manager Louis van Gaal, former Wales assistant coach Raymond Verheijen said on Thursday.

England left back Shaw, signed from Southampton in the close season for around £30 million ($A53.7million), may be out for a month with a hamstring problem and will definitely miss the start of the Premier League campaign at home to Swansea City on Saturday.

"LVG is tactically superior to most other coaches but planning & periodisation is not his strongest point: double sessions & muscle injuries," added the Dutchman, an expert on football fitness and a frequent critic of the methods of top coaches.

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"The Luke Shaw case is the climax of Manchester United's predictable pre-season injury crisis due to 'too much training too soon'."

Verheijen questioned why Shaw, 19, cut short his post-World Cup break and said he reported back for pre-season training too early.

Van Gaal accused the left back of being unfit on their pre-season tour of the United States.

"Luke Shaw had to do extra training to get fit. Who was in charge of his individual programme?," asked Verheijen.

"LVG's coaching skills & tactical superiority often compensate for this periodisation deficiency but there is clearly room for improvement."

Verheijen suggested Van Gaal's methods were responsible for a spate of injuries among the Dutch players before and during the 2014 World Cup in Brazil, a tournament where they finished third.

"During the World Cup preparation the Dutch players had to do frequent double sessions so not surprisingly the muscle injuries accumulated," he said.

"At Man United, in the first few weeks, LVG has applied the same approach and, as expected, with the same result.

"But who is gonna tell a top coach that he has to raise his bar in a certain area? The 'yes-men' in his coaching staff? Journalists? Who...?"

Last season Verheijen attacked the training methods of Van Gaal's United predecessor David Moyes after Dutch striker Robin van Persie was injured.

He has also been critical in the past of top managers like Roy Hodgson, Roberto Mancini and Chris Coleman.